Since a temperature difference occurs to a each component frame while a pressing machine is used, the die height is changed, and when high product precision is required, the change in die height has a large influence on the product precision. Recently, there are more and more products that require very high product precision, and this problem becomes important. For such a change in die height, a die height adjusting apparatus is conventionally proposed, and the one disclosed in, for example, Japanese Utility Model Application Publication No. 3-29036 is known. FIG. 7 is a block diagram of a die height adjusting apparatus described in Japanese Utility Model Application Publication No. 3-29036.
In FIG. 7, a slide 4 is connected to a plunger 19 operated in a vertical direction via an adjusting screw 41, and by rotating the adjusting screw 41, the position of the slide 4 is made adjustable with respect to the plunger 19. A worm wheel 78 is concentrically fixed to the adjusting screw 41, and a worm 79 is meshed with the worm wheel 78. Two ratchet wheels 81 and 82, both having a number of claws, each of which is in an unequal-sided angled shape where one side of the claw is a catching surface, are fixed to a shaft of the worm 79, with the catching surfaces of the claws on the respective ratchet wheels facing in the opposite direction to each other. The tip ends of piston rods 85 and 86 of cylinder devices 83 and 84 oppose the side of the catching surfaces of the claws of the respective ratchet wheels 81 and 82 in the extension direction of the piston rods 85 and 86. Further, cylinder chambers of the cylinder devices 83 and 84 are connected to a fluid pressure source 89 such as a reservoir via solenoid valves 87 and 88.
However, in the above-described die height adjusting apparatus disclosed in Japanese Utility Model Application Publication No. 3-29036, the ratchet wheels 81 and 82 are driven in normal and reverse rotation by the cylinder devices 83 and 84 to rotate the adjusting screw 41 via the worm 79 and the worm wheel 78, and therefore, responsiveness is not so good. Consequently, the positioning precision, at the time of adjustment of the die height, cannot be made so high, thus making it very difficult to apply the apparatus to the products requiring high precision. In addition, the die height adjustment requires much time, and therefore this arises the disadvantage that the apparatus cannot respond to press working at high stroke per minute of, for example, 300 SPM or more, when the die height adjustment is performed for each press stroke, during slide operation, and at the time when working is not performed.